I've used Polyscale Reefer White to touch up factory handrails and to paint my own (because I convert three-rail diesels to "fixed" pilots and install full-length handrails, I paint a lot of these). That's an acrylic paint, and I like it because it washes up with soap and water and brush marks are imperceptible on small details like handrails. It does dry to a flat finish, but for small touch up that probably won't be noticeable.
In tight spots, use a piece of masking tape or an index card between the hand rail and the body or pilot to keep the paint off those.
I have also used craft store acrylic paints to do handrails and other details. Once you get the hang of thinning them with a little water, they are useful. I think that's how I did the homemade handrails and the red MU hose on this model: